Assets management is required in many fields. A very important function of asset management is to locate and track the assets. For example, in warehouses, accurate location of the stored assets accelerates carrying goods in and out. In libraries, assistants need to know the location of the books and be aware timely when some books are placed on the wrong shelf. In retail stores, shop assistants need to find out the goods placed at the wrong places by customers and put the goods back to the position where they ought to be. If these problems of asset management can be well solved, the efficiency, responsivity and customer satisfaction will be improved and the cost of time and human resources will be reduced.
Recently, the enthusiasm for RFID solutions has been ignited by the RFID 2005 mandatory deadline set by Wal-Mart and US Department of Defense. Because of the requirement by the market and the many merits in technology, RFID solutions are increasingly regarded as the best choice for assets management. In a common configuration, an RFID tag is attached to each asset, some RFID readers are installed at some fixed key positions to monitor if any RFID-tagged assets passing by. The backend computer system collects the information from RFID readers to infer where the asset is. Some basic scenarios are, the RFID readers are installed at the check-in and check-out counters in libraries to monitor books' lending out and returning in, or the RFID readers are installed at the gate of warehouse and the check-out counters in retail stores to track the goods in and out. All these configurations can provide coarse-granularity assets management ability for tracking if an asset is in a room, but they can not provide the information regarding the exact location of the asset within the room.
In order to be capable of positioning an asset in a physical space, some approaches have been proposed. A typical approach is shown in FIG. 1, in which a matrix of RFID readers are installed at some fixed positions. The position of each reader is recorded in advance. When an RFID-tagged asset is within the read range of an RFID reader, it can be detected by the reader. After the reader notifies the backend computer system of its detecting this RFID tag, the asset is associated with the location covered by the reader.
The approach shown in FIG. 1 can locate an asset within a physical space; however the cost of this solution is very high. For RFID tags and readers commonly used in warehouses, libraries and retail stores, the read range of an RFID reader is about 1-1.5 meters. Even if we approximate the coverage area of a single reader as big as 3*3=9 m2, for a warehouse of 1000 m2, there would require more than 100 readers installed to locate an RFID-tagged asset. This is very expensive because:                1. RFID readers are quite expensive per se; an RFID tag may cost less than 50 cents, but a common reader with a read range of 1 meter could cost several hundred dollars; and        2. The workload for installing the matrix of the readers, providing power supply and connecting them to the backend computer system is very heavy and requires many deployment skills.        
Thus, RFID technology provides an easy-to-use way for assets management, however, due to cost consideration, fixed reader solutions can only provide coarse-granularity location information of an asset currently.
U.S. patent application US 2003/0214387 and U.S. Pat. No. 6,693,512 respectively describe a system for locating assets by using a portable apparatus, but because they are only used to seek particular objects and do not contain a locating module, they can only seek a particular object once, and cannot associate all assets with their positions. Whenever a user needs to seek a particular asset, he/she must search for the entire physical space over again.
U.S. patent application US 2003/0236590 describes an RFID coordinate system for recognizing the position and direction of a mobile unit, in which an RFID reader is installed on a mobile robot and RFID tags representing the position of themselves are deployed at a plurality of fixed positions in the robot moving area. The position of the robot is obtained by the RFID reader detecting the RFID tags. However, this is only “one-way” positioning, in which the RFID reader detects only which position the robot moved to, but do not detect which assets are placed at the position.